The purpose of this project is to determine the distribution of calcium in cellular organelles and the effects of physiologic stimuli on the distribution of calcium and the morphology of calcium-containing organelles. It is particularly concerned with the question whether the endoplasmic reticulum (ER) is a calcium uptake system and whether calcium can be released from the ER in response to surface membrane stimuli such as specific ligand-receptor interactions. Tissue is prepared by rapid freezing or by modified fixation procedures designed to retain Ca in situ. Ca is identified in sections by electron-probe x-ray microanalysis. Early stages of this study have been devoted to refinement and testing of the methods. Results thus far with macrophages and fibroblasts suggest that similar to muscle and nerve, the endoplasmic reticulum in these cells contains calcium, and furthermore, it forms morphologically specialized oppositions with the surface membrane similar to those between the surface and sarcoplasmic reticulum membrane at triads in muscle. Another class of organelle also contains calcium: organelles whose membranes circulate through the cell surface via endocytosis and exocytosis.